Field of the Invention
The present invention relates to glass strands comprising an electrically conducting coating.
It also relates to the electrically conducting coating composition used to coat the said strands, to the process for the manufacture of these strands, to the reinforcing structures formed from these strands and to the composite materials including these strands.
Description of the Background
Reinforcing glass strands are conventionally prepared by mechanically drawing molten glass streams flowing by gravity from the multiple orifices of bushings filled with molten glass, to form filaments which are gathered together into base strands, which strands are then collected.
During the drawing, and before they are gathered together into strands, the glass filaments are coated with a sizing composition, generally an aqueous sizing composition, by passing over a sizing member.
The role of the size is essential in two respects.
During the manufacture of the strands, it protects the filaments from the abrasion resulting from the rubbing of the latter at high speed over the members of the process, acting as a lubricant. It also makes it possible to remove the electrostatic charges generating during this rubbing. Finally, it gives cohesion to the strand by providing bonding of the filaments to one another.
During the use for the purpose of producing composite materials, the size improves the wetting of the glass and the impregnation of the strand by the matrix to be reinforced and it promotes the adhesion between the glass and the said matrix, thus resulting in composite materials with improved mechanical properties.
The glass strands in their various forms (continuous, chopped or milled strands, mats, grids, fabrics, and the like) are commonly used to effectively reinforce matrices of varied natures, for example thermoplastic or thermosetting materials and cement.
Generally, the glass strands are rendered conducting by the application of a coating based on particles capable of conducting the electrical current. The coating is obtained by depositing, on the strands coated with the size, the conducting particles in dispersion or in suspension in an aqueous medium and by removing the water by heating at an appropriate temperature.
The compositions known for the preparation of the abovementioned coating use, as conducting particles, graphite, carbon black or organometallic compounds capable of decomposing to give metals under the action of heat, if appropriate by introducing a carbon-comprising compound capable of giving carbon by thermal decomposition into the composition (U.S. Pat. No. 3,269,883) or into the size (U.S. Pat. No. 3,247,020).
In U.S. Pat. No. 4,090,984, use is made of a semi-conducting coating composition comprising at least one polyacrylate emulsion, one carbon black dispersion and one thixotropic gelling agent. The carbon black dispersion represents 20 to 40 parts per 100 parts of the composition. In example 1, the content of carbon black is equal to 11.9% by weight of the solid matter present in the composition.
In U.S. Pat. No. 4,209,425, the coating composition comprises conducting particles, in particular made of graphite or of carbon, at least one surfactant, one thixotrophic gelling agent and optionally one organosilane coupling agent and/or one antifoaming agent. The content of conducting particles in the composition is between 5 and 15% by weight of solid matter of the composition.
With the compositions which have just been mentioned, the content of conducting particles in the final coating remains low, resulting for this reason in a low level of electrical conductivity.
Recent times have seen the appearance of novel materials incorporating glass strands which exhibit high electrical conductivity properties and which can for this reason be heated by the Joule effect. These materials include in particular composite materials with an organic matrix, of the thermoplastic or thermosetting polymer type, or cement matrix, in which materials the abovementioned strands also play a reinforcing role.
The improvement in the electrical conductivity must not be made at the expense of the other properties. As regards the composite materials, it must in particular be kept in mind that the strands are above all intended to reinforce matrices and consequently they must exhibit all the qualities for this.
In particular, the conducting coating must:                provide bonding of the filaments to one another and also bond the strands so as to obtain acceptable or improved mechanical properties when composite materials are concerned,        protect the glass strands from the mechanical assaults which occur when the reinforcing structures are employed on building sites,        protect the glass strands from chemical corrosion and from assaults related to the environment, so as to provide satisfactory durability, and        provide good bonding with the polymer matrix to be reinforced, that is to say render the strands and the matrix compatible.        